1. Field of the Invention
This invention relates to heat developable light-sensitive materials and particularly to heat developable light-sensitive materials containing a certain reducing agent resulting in reduced thermal fog of the background, increased whiteness and high stability to normal room illumination.
2. Description of the Prior Art
The most widely used photographic process is a process in which silver halides are employed since such a photographic process has superior photographic properties such as sensitivity and gradation to other photographic processes such as an electrophotographic process and a diazo process. There are, however, disadvantages in the silver halide photographic process. For example, the silver halide light-sensitive element must be developed in an aqueous developing solution after exposure and the thus developed light-sensitive element must be further subjected to several aqueous solution processings such as stopping, fixing and stabilization in order to prevent the thus formed image from discoloring or fading and to prevent the undeveloped white areas in the image (background) from darkening. These solution processings are time-consuming and troublesome, and the chemicals employed in these processings are hazardous in handling and stain the worker's body and clothes and the processing room and further may give rise to water pollution if the solutions are discharged without treatment.
Therefore, a light-sensitive element which utilizes high speed silver halides can be processed in a dry manner without using processing solutions and which is, moreover, capable of forming a stable image with minimized discoloration in the background areas thereof under normal room illumination has been desired.
To achieve such, various efforts have been made. For example, German Pat. Nos. 1,123,203 and 1,174,157 disclose that heat development of a silver halide light-sensitive element is possible by incorporating therein a 3-pyrazolidone type developing agent. German Pat. No. 1,175,075 discloses that the heat-developability of the element can be promoted by incorporating therein a material capable of forming water, and German Pat. No. 1,003,578 discloses the additional incorporation therein of a fixing agent for the silver halide. According to the above-described techniques, however, the silver halide itself remaining in the element after the dry processing would never be completely stabilized against light by a dry process, that is, the former three patents do not described a dry fixing step, and in the light-sensitive element described in the last patent it can be easily appreciated that the copresence of a developing agent (a reductant) and a fixing agent during storage would give rise to an undesirable reaction, which makes the element not very practical. This has been confirmed experimentally.
At present the most successful light-sensitive element capable of forming a photographic image by a dry processing method is a heat developable light-sensitive element employing a composition comprising a silver salt of an organic acid, a small amount of a light-sensitive silver halide and a reducing agent, e.g., as described in U.S. Pat. Nos. 3,152,904 and 3,457,075. In this light-sensitive system the silver halide, which remains in the element after development, is not stabilized against light and allowed to be discolored by light. Nevertheless the element provides results as if it were stabilized, because the silver halide is used in a slight amount and most of the silver source is a white or light-colored organic silver salt which is stable and hardly blackened by light. Thus even if a small amount of silver halide is discolored by light, the slight discoloring does not give rise to any difficulties visually since the element, on the whole, still appears white or only slightly colored. The light-sensitive element is stable at normal temperature, but when it is image-wise exposed and heated to a temperature, usually, above about 80.degree. C., preferably above 100.degree. C., the organic silver salt oxidizing agent and the reducing agent which are present in the light-sensitive element undergo oxidation-reduction reactions due to the catalytic action of the exposed silver halide present in the vicinity thereof to form silver. By this reaction, the exposed areas of the light-sensitive layer are rapidly blackened so that a contrast is formed between the exposed areas and the unexposed areas (background), that is, an image is formed.
The present invention relates to an improvement in the above-described heat developable light-sensitive materials, particularly in the reducing agents incorporated therein, and an object is to employ a reducing agent giving rise to improved properties. In order for reducing agents to be an effective constituent of heat developable light-sensitive materials, the reducing agent must be capable of reducing organic silver salts when heated in presence of exposed silver halide as a catalyst, that is, they must exhibit a heat developable property. Reducing agents which have already been proposed in the art include substituted phenols, substituted mono- and bis-naphthols, di- and poly-hydroxybenzenes, di- and polyhydroxynaphthalenes, hydroquinone monoethers, ascorbic acid and the derivatives thereof, 3-pyrazolidones, pyrazoline-5-ones, reducing saccharide, p-phenylenediamine and the derivatives thereof, reductones, kojic acid, hinokitiol, hydroxylamines, hydroxytetronic acid, hydroxytetronamides, hydroxaminic acids, sulfohydoroxaminic acids, hydrazides, indan-1,3-diones, p-oxyphenylglycine and so on. A number of patents are concerned with these reducing agents, for example, U.S. Pat. Nos. 3,152,904; 3,457,075; 3,531,286; 3,615,533; 3,679,426; 3,672,904; 3,751,252; 3,751,255; 3,782,949; 3,770,448 and 3,773,512: German Patent Application OLS No. 2,031,748: and Belgian Pat. No. 786,086.
However, these reducing agents undesirably cause the exposed area of a light-sensitive material to become black in the process of heat development, namely a large amount of thermal fog is produced by these reducing agents. Even if heat-development is conducted within a comparatively short time, light-sensitive materials containing such reducing agents also have light yellowish brown stains in the background. Moreover, the background of these processed light-sensitive materials become yellowish brown due to diffuse room light.
A solution of the above-described problems inherent in conventional techniques has been an important subject in the art of heat developable light-sensitive materials.